The ubiquitous AIM-9 Sidewinder
is without doubt the most important heatseeking missile of the last
three decades, seeing service in every engagement between Western
powers
and their adversaries since the 1950s. Shamelessly copied by the
Communists as the K-13/AA-2 Atoll, the Sidewinder has had a profound
influence on the design of modern heatseekers and is much the yardstick
against which such missiles are judged today.

The
collapse of the agreement between the US and EEC nations on the ASRAAM,
the planned successor to the AIM-9, has seen further life injected into
the humble Sidewinder, with the US at the time of writing electing to
continue the development and production of the weapon for its own
services.

The
AIM-9 has had a colourful history and has evolved considerably since
the
first of its kind left a missile rail in the distant fifties. It is
story of technical ingenuity as much as operational application and
more
than anything underscores the fundamental soundness of the basic
airframe and system design, which has seen ongoing evolutionary
development since the fifties.

The Beginning - the AIM-9B
The AIM-9 traces its earliest ancestry to the US Naval Weapons Center
at China Lake, in the Mojave Desert. THe NWC initiated, in the early
fifties, a program to design a heatseeking air intercept missile for
the
intercept of bombers by naval interceptor aircraft, until then armed
with either .50 cal or 20 mm guns. The fledgling missile was aptly
named
after the Sidewinder, a desert rattlesnake which detects its prey by
sensing the animal's heat emissions.

The
result was a compact lightweight cruciform canard weapon, which used a
solid propellant rocket motor, a fragmentation warhead and an uncooled
optical seeker.

The
Sidewinder's seeker used an ingeniously clever optical arrangement,
with
a Cassegrainian mirror fitted with a tilted secondary mirror. The
secondary mirror rotated in unison with a reticle, projecting the whole
instantaneous field of view of the mirror through the reticle onto a
filter/detector assembly. Because the mirror secondary was tilted,
rotating it about the missile's axis swept the cone of the mirror's
field of view about the missile's axis in a fashion analogous to a
conical scanning radar seeker (see diagram).

The
missile used a 2.5" glass dome nose window, transparent to 1 micron
band
radiation, providing with the gimballed seeker for a 25 degree seeker
field of view. The mirror assembly provided a 4 degree instantaneous
field of view (IFOV), projected on to a PbS (Lead Sulphide) uncooled
detector. Because of the design of the optical system, the AIM-9B was
strictly a tail aspect weapon, as it was blind to anything cooler than
a
tailpipe. The modest 11 deg/sec seeker tracking rate limited the weapon
to non-maneuvering targets. All seeker electronics were built from
vacuum tubes. A hot gas generator provided actuator power for the nose
canards, and was limited to a 20 sec burn duration before exhaustion.
Unlike other missiles of the day, the Sidewinder did not employ active
roll stabilisation (via gyros and differential control input), instead
employing rollerons, ie slipstream spun metal discs embedded in the
trailing edge of the wingtips, which acted as four tiny gyros
stabilising the missile mechanically. The engineer who thought of that
certainly earned his paycheck.

The
AIM-9B used a fragmentation warhead triggered by a passive infrared
proximity fuse. The Thiokol Mk.17 solid propellant rocket delivered
8,200 lb-sec of impulse with a burn duration of 2.2 seconds.

While by modern standards the AIM-9B is a very limited weapon, it had
no
serious competitors in its day and was soon adopted by the USAF and
NATO
as a standard weapon, with no less than 40,000 guidance units built by
Ford Aerospace, the prime contractor. The RAAF also adopted the
missile,
fitting it to the CAC Avon-Sabre, and subsequently the Mirage.

NATO
rounds were mainly built by West Germany's FGW, who evolved an improved
subtype designated the AIM-9B-FGW Mod.2. This AIM-9B used solid state
electronics, carbon dioxide seeker cooling, a new nose dome and better
optical filtering, the latter providing for much better seeker
sensitivity.

The
Sidewinder was by the early sixties the principal heatseeker in Western
service and as such first drew blood over North Vietnam, there used by
the USAF and USN. Its early combat record was not spectacular, as the
seeker performance limitations were exacerbated by the poor reliability
of the tube electronics and the inexperience of its users, who until
then trained for intercepts rather than dogfights. Kill probabilities
were in the tens of percent, very sensitive to how well the launch
aircraft was positioned. Designed to intercept lumbering bombers, the
AIM-9B was ill suited to knife-fights with MiG-17s at low level. Its
launch load factor limit of 2G hampered aircrew, while its seeker very
often locked on to the sun or clouds, subsequently sending the missile
ballistic. The range limit of 2.6 NM meant that the launch aircraft had
to be quite properly positioned for a shot, and the pilot very careful
about closure rate and range.

Nevertheless, no less than 28 MiGs were killed for 175 launches between
1965 and 1968, by USAF F-4C/D aircraft, an aggregate P[k] (kill
probability) of 16%.

The Navy Sidewinders -
AIM-9D, G and H
The US Navy recognised the limitations of the AIM-9B during the early
fifties, and soon proceeded with the development of a follow-on subtype
with improved performance. Thus was born the first of a series of
dedicated naval Sidewinders, a split in the family tree which persists
to this very day.

The
most important change in the AIM-9D was the use of a Nitrogen cooling
system for the PbS detector element, coupled to a redesigned optical
system. The new optical system retained the tilted cassegrain of the
earlier subtype, but was more compact, fitting into a ogival nose
section, and spun at a higher frequency of 125 Hz, rather than the 70
Hz
of the B-model. The IFOV was reduced to 2.5 degrees, while the seeker
FOV was slightly increased to beyond 25 degrees. The glass nose dome
was
replaced by a much smaller Magnesium Fluoride dome, which provides
better transparency to longer wavelength (cooler) infrared emissions.

The
Nitrogen coolant was contained in a 6 litre bottle in the Navy LAU-7
launcher, and provided for 2.5 hrs of seeker cool down.

The
seeker changes provided a higher target tracking rate of 12 deg/sec,
and
this was further assisted by an improved actuator system, which
delivered up to 100 lb.ft of torque. Both of these measures improved
missile manoeuvrability, while a longer gas generator burn provided for
a 60 second usable flight time. The rocket motor was changed to a
Hercules Mk.36, with more impulse and longer burn.

Changes were also introduced to the fusing, with the option of an
infrared fuse or a radio-frequency proximity fuse, which fired a new
continuous rod warhead. Continuous rod warheads have a casing of
lengthwise rods welded together at alternate ends, on detonation the
rods expand into a circle about the missile before breakup, upon which
a
torus of fragments is produced about the axis of the weapon. These rods
are like knives which chop into the skin and structure of the target.

In
comparison with the AIM-9B, the AIM-9D had a much wider engagement
envelope, was more manoeuvrable and hence offered a better P[k]. The
AIM-9D entered production in the late fifties, and Ford Aerospace built
about 1,000 systems.

The
AIM-9D was succeeded by the very similar AIM-9G, which employed SEAM
(Sidewinder Extended Acquisition Mode), a facility which slews the
optics through a search pattern to acquire the target (most likely
using
a rosette scan), or allows slaving of the optics to radar or a helmet
sight. The latter was used extensively by Navy F-4s during the
seventies, before the deployment of the F/A-18.

The
AIM-9D/G was used extensively by the Navy in Vietnam, accounting for
many kills of manoeuvrable adversaries such as the nimble MiG-17 and
MiG-19, engaging Navy aircraft at low altitude. Most Navy kills were
scored with this weapon.

The
latter phase of the Vietnam war, the Linebacker campaigns, saw the
first
application of the subsequent naval Sidewinder, the AIM-9H. The Hotel
saw some radical changes resulting from experience with the D/G, which
suffered reliability problems due the intolerance of vacuum tubes to
repeated 20 ft/sec sink rate recoveries on aircraft carrier decks. The
AIM-9H was the first solid state Sidewinder, with the complete guidance
package built with semiconductors. In redesigning the electronics, the
G
optical system was essentially retained, but the tracking rate was
further increased, to complement the more powerful 120 lb.ft actuators.

While few of the AIM-9H were fired in combat due shortages of supply,
they are reported to have scored a much higher kill rate per launch
than
any other Sidewinder in the campaign. Over 3,000 were built.

The AIM-9H was by far the best of the early
Sidewinders and distinguished itself in Vietnam achieving the best kill
rate of any missile in the campaign. Using a solid state seeker with a
Nitrogen cooled Lead Sulphide detector, and a more powerful actuator
system, the AIM-9H was the most manoeuvrable of its kind. The
subsequent
AIM-9L was directly derived from the AIM-9H.

The Air Force Sidewinders -
AIM-9E, J and N
Unlike the USN which was focussed on the tactical air battle, the USAF
had diluted its resources into several AAM programs and thus lagged in
the development of their own Sidewinder subtypes. Vietnam saw the
AIM-9B
perform questionably, and the USAF sought improvements to the design to
enhance performance against fighter type targets. The result was the
AIM-9E.

The
AIM-9E saw the adoption of a similar low drag nose to the Navy
subtypes,
but using a conical rather than ogival profile, a distinguishing
feature
of this family to this very day. A Magnesium Fluoride dome was adopted,
a more compact optical assembly was used, with a faster 100 Hz reticle
rate, and a 16.5 deg/sec tracking rate. The canards were changed to the
characteristic squared tip double delta planform, adopted to improve
canard behaviour at higher angles of attack. Significant changes were
made to the internal wiring harnesses.

The
most significant design change was the adoption of a cooling for the
PbS
detector element, the USAF opting for Peltier thermoelectric cooling.
This arrangement has the advantage of unlimited cooling time on the
launch rail, subject only to the availability of electrical power. The
seeker improvements expanded the weapon's acquisition envelope and
increased its P[k], although not dramatically. Over 5,000 rounds were
rebuilt from AIM-9Bs.

The
AIM-9E was deployed to SEA in the late sixties, and saw considerable
use
by the USAF. Because most USAF engagements were flown at higher
altitudes, the weapon was used far less often than the Sparrow, and at
short ranges the USAF F-4E nd F-105D/F/G tended to rely on the internal
M-61 gun, therefore the AIM-9 accounted for only 14% of USAF kills.
Arguably this environment produced less pressure for improvements in
USAF AIM-9 performance, resulting in the use of less capable versions
in
comparison with the Navy, whose F-4Bs were armed solely with missiles.

An
interesting idiosyncrasy of the Vietnam period Sidewinders was how they
killed their targets, as they invariably pursued their targets and in
most instances flew up the tailpipe, bisecting the MiG aft of the wing
upon warhead detonation. Much interesting gun camera footage exists.

AIM-9J launch from US Air Force
F-4D Phantom (US Air Force image).

The
follow-on version to the AIM-9E was the AIM-9J, which was rushed into
the SEA theatre in July, 1972. The Juliet model saw incremental
improvements to the AIM-9E design, with hybrid electronics using a mix
of solid state and tube technology, and an improved control system
using
a longer burning gas generator for a 40 sec flight time, and more
powerful actuators delivering 90 lb.ft torque to the canards. 6,700 of
this subtype were eventually built or rebuilt from AIM-9Bs.

In
1973, Ford began production of an enhanced AIM-9J-1, later redesignated
the AIM-9N. The November model employed a similar configuration to the
Juliet, but the three main printed circuit boards were substantially
redesigned to improve seeker performance. Close to 7,000 of this
version
were built. The Hercules-Aerojet Mk.17 rocket motor was retained.

AIM-9 EARLY SUBTYPE
COMPARISON TABLE

Subtype

AIM-9B

AIM-9D

AIM-9E

AIM-9G

AIM-9H

Service

Joint

USN

USAF

USN

USN

Seeker
Design
Features

Origin

NWC

AIM-9B

AIM-9B

AIM-9D

AIM-9G

Detector

PbS

PbS

PbS

PbS

PbS

Cooling

Uncooled

Nitrogen

Peltier

Nitrogen

Nitrogen

Dome
Window

Glass

MgF2

MgF2

MgF2

MgF2

Reticle
Speed
[Hz]

70

125

100

125

125

Modulation

AM

AM

AM

AM

AM

Track Rate
[deg/s]

11.0

12.0

16.5

12.0

>12.0

Electronics

thermionic

thermionic

hybrid

thermionic

solid state

Warhead

blast/fragment

continuous rod

blast/fragment

continuous rod

continuous rod

Fuse

Passive-IR,

Passive-IR/HF

Passive-IR

Passive-IR/HF

Passive-IR/HF

Powerplant
Specifications

Manufacturer

Thiokol

Hercules

Thiokol

Hercules

Hercules Bermite

Type

Mk.17

Mk.36

Mk.17

Mk.36

Mk.36 Mod 5, 6, 7

Launcher

Aero-III

LAU-7A

Aero-III

LAU-7A

LAU-7A

Missile
Dimensions[ft]

Length

9.28

9.4

9.84

9.4

9.4

Span

1.83

2.06

1.83

2.06

2.06

Weight[lb]

155.2

195.1

164.2

191.8

186.3

Author's note:

This table was
compiled from a wide range of sources, many of which do not
specify the exact conditions under which the stated
performance figures apply. Therefore caution must be exercised in
interpreting the figures. Since newer variants are still
current in service, detailed figures for these are hard to acquire.

The All Aspect Sidewinders -
AIM-9L and M
The lessons of Vietnam and the Yom Kippur clearly indicated the
limitations of the established Sidewinder subtypes, which did not
perform well at low altitudes due background infrared emitted by the
earth's surface and reflected by clouds, and were limited to tail
hemisphere engagements due to the spectral range of the PbS detector
element used. The need for all aspect capability against agile targets
dictated changes in both detector material and proximity fuse, as the
passive IR fuses used to date relied on sensing the target's exhaust at
close range. Much debate took place in the mid-seventies as to the
successor to the Sidewinder, with a USN proposal for a modified AIM-9H
eventually adopted as offering the lowest risk. A West German proposal
to fit a fully gimballed seeker to the AIM-9H airframe, creating the
ALASCA (all aspect capability) missile was dropped.

The
AIM-9L is essentially an AIM-9H with a new optical system, new fuse and
new cooling system. The Cassegrain system of the H was retained, but a
new FM reticle was adopted, necessitating some fundamental changes to
the guidance electronics. A new 4 micron band Indium Antimonide (InSb)
detector was used, enclosed with an optical filter in a cryogenic
container. This optical system allows acquisition and tracking of
targets from all aspects, due the longer wavelength sensitivity of the
InSb, with the filter employed to reject shorter wavelengths. Argon gas
is used to cool the detector, with the coolant tank embedded in the
missile's seeker to allow use with arbitrary physically/electrically
compatible launchers.

The
FM reticle provides superior performance by reducing the effect of a
target's increasing size with decreasing range on the seeker error
signal output, a factor which can affect the behaviour of AM seekers,
while providing the potential for better countermeasures rejection. In
addition, design changes were adopted to bridge the dead zone about the
missile's immediate boresight, a characteristic of conventional reticle
seekers. To widen the manoeuvre envelope, lambda compensation is
employed, a technique which prevents the seeker from reaching its
angular limit during the early phase of its flight. If a target can
force the seeker past its angular limit, lock is broken and the missile
lost.

The
control actuators of the AIM-9H were retained, but the canards were
redesigned to the characteristic pointed tip double delta. The
combination of new seeker and canards results in much better manoeuvre
performance than any earlier subtype, while the new detector allows
acquisition of targets from any aspect at substantially greater ranges.

The
need to engage dogfight targets required a better fuse and warhead. The
AIM-9L was the first subtype to introduce an active laser proximity
fuse, the DSU-15A/B. This device uses a group of Gallium Arsenide
(GaAs)
solid state lasers which radiate a spoke-like pattern about the
missile,
each laser is paired to a Silicon (Si) photodiode. The lasers emit a
stream of pulses, which are reflected by an object which enters the
fuse
pattern, and sensed by the photodiodes, triggering warhead detonation.
The scheme is highly resistant to jamming.

The
warhead is also more lethal, using an annular blast arrangement with
two
layers of rods, which are specifically designed to tumble at high speed
to ensure that their effectiveness is not reduced by impact angle on
the
target, a limitation of the conventional continuous rod scheme.

The
Lima entered service in the late seventies and first drew blood in
1982,
used in both the Falklands campaign and the Bekaa Valley air battle. In
both campaigns the weapon was a star performer, achieving kill
probabilities in excess of 80%. Royal Navy Harriers and Israeli F-15s
slaughtered their opponents in head-on shootouts, the hapless
Argentines
and Syrians never devising suitable evasive manoeuvres. Over 5,500
rounds were built and were sold only to trusted allies, with licenced
production by Bodenseewerk in Germany and Mitsubishi in Japan.

The AIM-9M is an improved AIM-9L, with a low smoke
motor, better guidance and counter-countermeasures capability. The
AIM-9M is the principal dogfight missile used by frontline Western air
forces, and also equips the FAF's F/A-18 force. The AIM-9M was
responsible for all of the 10 Sidewinder kills scored during the Gulf
conflict, with most kills scored by the longer ranging Sparrow due the
Iraqis' reluctance to engage in dogfights.

The
Lima was followed in production in 1982 by the AIM-9M, which is
essentially an improved AIM-9L. The Mike has improved background
rejection, counter-countermeasures capability and a low smoke motor to
reduce the visual signature of the inbound weapon. The latest subtypes
due for deployment are the AIM-9M-8 (Navy) and AIM-9M-9 (USAF). The
AIM-9M is the RAAF's standard dogfight AAM, carried by the F/A-18 and
F-111.

The
AIM-9M was deployed in large numbers during last year's Gulf war, but
only several kills were achieved as the Iraqi air force preferred to
let
itself be taken out with six o'clock Sparrow shots. Were they to have
done the proper thing and put up a serious fight, the the statistics
may
have been different, albeit the eventual outcome identical.

The Export Sidewinders - AIM-9P
While the AIM-9L fulfilled the role of the frontline all aspect
dogfight
missile, a need still existed for a second tier weapon for use in less
demanding situations, and also suitable for export to less than
absolutely trusted allies. This requirement was fulfilled by the AIM-9P
family, derivatives of the AIM-9J/N.

The
AIM-9P-2 and P-3 were introduced in the mid seventies and use improved
guidance electronics, a new rocket motor and an active optical fuse.
While not receiving the publicity of the AIM-9L, the success of the
weapon is testified to by the fact that no less than 21,000 have been
built, with substantial numbers in the USAF inventory.

AIM-9P-4 engages a QF-102A
drone
(US Air Force).

The AIM-9P is a USAF sponsored development of the AIM-9J/N
family, to provide a missile for use in less demanding applications.
The AIM-9P has evolved through the P-2, P-3 to the all aspect P-4, and
the P-5 with additional counter-countermeasures capability. Large
numbers of various AIM-9P subtypes are in use with the USAF and many
export customers. The missile retains the conical nosecone and
characteristic double delta canards first used in the Vietnam era USAF
AIM-9E.

AIM-9P-3
on
F-15 Eagle (US Air Force)

The
AIM-9P-4 is an incremental development of the AIM-9P-3, with an all
aspect seeker using some of the technology developed for the AIM-9L. In
comparison with its cousin, it is less agile but still a very effective
missile. The AIM-9P-5 is further improved by the addition of a
counter-countermeasures capability. The wide range of types which can
carry the P-3/4/5 suggest that the gas coolant is carried on board, as
with the L/M.

The Radar Homing
Sidewinders - AIM-9C and AGM-122A
The
Sidewinder family has spawned but one radar guided subtype, the US Navy
AIM-9C semi-active radar homing missile. This weapon was designed to
arm
the lightweight F-8 Crusader with an all weather missile, and used a
conically scanning semi-active seeker. No records exist as to the
missile's combat record and it was all but forgotten until the mid
eighties, when stocks of several hundred rounds were resurrected from
storage to fulfill a Marine Corps requirement for a lightweight
Anti-Radiation Missile for suppression of air defences.

The
AGM-122A Sidearm is essentially a rebuilt AIM-9C, in which the narrow
band semi-active seeker electronics have been modified for the much
greater bandwidth required to home in on a wide range of radars. In
addition, the later DSU-15 active fuse was used, while the Mk.17 motor
and WDU-17 warhead were retained. The control electronics were also
modified to command an immediate pop-up after launch at low level, to
provide for a dive attack on the offending emitter.

The
AGM-122A was developed by the NWC and subsequent remanufacturing
carried
out by Motorola, the status of the program is at this time unclear. The
weapon in no way compares with serious ARMs such as the HARM and ALARM,
and is easily seduced by countermeasures, but provides a useful means
of
suppressing unfriendly attention by systems such as the ZSU-23 or SA-8.
No reports exist on combat use, the weapon was built to equip Marine
AV-8s, A-4s and helicopters.

The Imaging Sidewinder - AIM-9R
The
AIM-9R is the latest production Sidewinder, using an imaging seeker
which is a fundamental departure from the established design. Developed
by the Naval Weapons Center, the AIM-9R uses a modified AIM-9M control
actuator, while retaining the fuse, warhead, motor, wings and canards
of
its predecessor.

The
imaging seeker is built around a focal plane array imaging device,
analogous to the CCDs employed in modern television cameras. A focal
plane array has a much greater instantaneous field of view than a
reticle seeker, and 'stares' at the target and its immediate
background,
tracking the target by means of a contrast lock similar to that
employed
by TV guided weapons such as Maverick or GBU-15. In this fashion, the
seeker can account for the background contrast and reject it, while
also
providing the potential to discriminate between multiple targets and
countermeasures such as flares. Conventional pulse jammers have no
effect.

The
WGU-19 seeker uses a three gimbal stabilised platform mounting a
visible
band focal plane array device, most likely a 256 x 256 element InSb
array, or a higher resolution PtSi device on a Peltier cooled
substrate,
to provide coverage down to the 4 micron band. The video signal
produced
by the array is then digitised and processed by a software programmable
digital image processor, which tracks the targets and feeds the
autopilot with data so it can send steering commands to the control
actuators.

The imaging AIM-9R is the latest naval Sidewinder,
using the airframe, fuse and motor of the AIM-9M with a new digital
imaging seeker. The new seeker employs a focal plane array imaging
device effective to visual wavelengths, mounted on a gimballed
stabilised platform. The use of the imaging seeker has provided a vast
improvement in target detection range, off boresight angle, rejection
of
background and ability to selectively aim for vulnerable areas of the
target. Imaging seekers are immune to jamming techniques effective
against reticle seekers.

The
AIM-9R provides a major increase in target acquisition range over
established subtypes, with much better tracking performance, and the
ability to reject both background terrain and clouds. The total field
of
view of the seeker is much greater, allowing acquisition of
off-boresight and manoeuvring targets, while the software provides for
intelligent selection of an aimpoint when impacting a target. Loral
Aeronutronic, the manufacturer, claim effective counter-countermeasures
capability against known and postulated jamming or seduction techniques.

At
the time of writing (1994) the AIM-9R had successfully flown live
firing
trials, and was in pilot production, with its long term future unclear
due ongoing defence cuts.

The Canard-less Sidewinder and
the AIM-9X
The
follow on to the AIM-9M/R in joint service use is the AIM-9X, at this
time still very much a development exercise intended for deployment in
the late 1990s. (Author's note: this feature article predates the final
US decision on the AIM-9X configuration, which is to use an SBRC 128^2
InSb FPA seeker, digital autopilot, fixed forward canards, and
steerable
tail surfaces and thrust vectoring, retaining the original motor).

The
AIM-9X has yet to be firmly defined, but reports indicate the weapon
will probably employ tail control with smaller or no canards. Two
airframe geometries will be used, a conventional layout for use by
conventional aircraft, and a flat bottomed layout for conformal
carriage
by stealthy fighters. The flat bottomed airframe is carried upside down
and rolls itself upright after launch, a side benefit of this scheme is
the potential to use the shape for lifting purposes, if an active roll
stabilising system is used.

The
seeker proposed is a focal plane array, possibly coupled to passive
radar homing for use as an ARM or semi-active homing weapon. The former
arrangement is used for instance by the GD RAM point defence SAM,
interestingly enough itself using a modified AIM-9L/M seeker, and the
latest AIM-7 subtype, which also uses an AIM-9L/M seeker.

Raytheon, a second source for later model AIM-9s, have under USAF
sponsorship expended three years of effort in developing a tail
controlled canardless AIM-9 airframe. While most of the details are
classified, the weapon uses the standard 5" airframe of the existing
AIM-9, with no canards, and a tail control system using a set of small
11" span cruciform movable fins which are arranged in a 60 degree/120
degree pattern, unlike the symmetrical tail of the existing AIM-9.

The
tail control missile is much faster due lower drag, and has
demonstrated
in tests higher peak speeds and load factors. A Raytheon designed
digital autopilot was fitted, providing for active roll control and
reportedly resulting in better stability and manoeuvrability than the
established design.

Live
firing tests carried out by the USAF 3245th Test Wing at Eglin in
Florida during the 1990/91 period saw eight launches, of which five
were
direct hits, one a would be lethal near miss, one a miss and one
control
system test flight without a target. These weapons employed standard
AIM-9M seekers.

Much
of the USAF's interest in the tail control Sidewinder stems from the
ATF
program, as the F-22 has small internal weapon bays unable to house the
standard Sidewinder.

The
Naval Weapons Center has also been working on modified AIM-9Ms under
the
Boa-M program, in which a standard airframe was fitted with smaller
AIM-9D canards and a smaller lower drag set of tail surfaces, and a
digital autopilot. At least one test firing is known to have been
carried out, to the detriment of a QF-86 drone.

Both
the USAF and USN programs are aimed at size and drag reduction, and
higher agility by the use of artificial stability.

AIM-9 LATE SUBTYPE COMPARISON
TABLE

Subtype

AIM-9J

AIM-9L

AIM-9M

AIM-9P-4/5

AIM-9R

Service

USAF

Joint

Joint

USAF

USN

Seeker
Design
Features

Origin

AIM-9E

AIM-9H

AIM-9L

AIM-9J/N

AIM-9M

Detector

PbS

InSb

InSb

InSb

Focal Plane Array

Cooling

Peltier

Argon

Argon

Argon

-

Dome Window

MgF2

MgF2

MgF2

MgF2

Glass

Reticle Speed
[Hz]

100

125

125

100

Focal Plane Array

Modulation

AM

FM

FM

FM

Focal Plane Array

Track Rate
[deg/s]

16.5

classified

classified

>16.5

classified

Electronics

hybrid

solid state

solid state

solid state

solid state

Warhead

blast/fragmention

Annular BF

Annular BF

Annular BF

Annular BF

Fuse

Passive-IR

IR/Laser

IR/Laser

IR/Laser

IR/Laser

Powerplant
Specifications

Manufacturer

Hercules/Aerojet

Hercules/Bermite

MTI/Hercules

Hercules/Aerojet

MTI/Hercules

Type

Mk.17

Mk.36 Mod.7,8

Mk.36 Mod.9

SR.116

Mk.36 Mod.9

Launcher

Aero-III

Common

Common

Common

Common

Missile
Dimensions[ft]

Length

10.0

9.5

9.5

10.0

9.5

Span

1.9

2.1

2.1

1.9

2.1

Weight[lb]

170.0

191.0

191.0

190.0

191.0

Author's
note:

This table was
compiled from a wide range of sources, many of which do not
specify the exact conditions under which the stated
performance figures apply. Therefore caution must be exercised in
interpreting the figures. Since newer variants are still
current in service, detailed figures for these are hard to acquire.

Cloned Sidewinders - The
K-13A/AA-2 Atoll
The Atoll traces its genealogy to the early AIM-9B. Numerous stories
exist on how the AIM-9 fell into communist hands. One suggests a
turncoat German officer drove some friendly GRU agents to a Luftwaffe
base and loaded an AIM-9 into a truck, subsequently transporting it
across the Iron Curtain. Another more probable story refers to a
dogfight between the Nationalist Chinese and Red Chinese over the
Formosa straights in the early sixties, during which an AIM-9B embedded
itself in the fuselage of a Shenyang F-6 fighter where it failed to
detonate.

Whichever way it transpired, the communists by the late sixties
deployed a missile which was very hard to distinguish from the AIM-9B.
Many USAF aircraft in Vietnam fell to sniping tail aspect GCI hits by
Atoll firing MiG-21s or F-8s, and the Atoll has since become the most
common heatseeking missile in Third World use.

Like the AIM-9B, the Atoll requires a skilled user to be effective at
its best, and therefore in Third World confrontations the weapon has
been of questionable usefulness. Well, at least it looks like a
Sidewinder !

The Sidewinder has stood the test of time, and spending 40 years at the
cutting edge is a tribute to its original designers, who can be justly
proud of their basic design. While its newer derivatives may look
different and use different guidance principles, they will all trace
their ancestry to the Naval Weapons Centre's original fifties program.
The Sidewinder must be acknowledged as the most successful heatseeking
missile design of all times.

Reticle
Seekers - A Brief Tutorial

The
reticle
seeker
is
the most common optical system design employed in
conventional heat seeking missiles. Invented by the Germans during the
latter phase of WW2, the reticle seeker provides a means of using a
single detector element to produce an error signal in rectangular
coordinates, with respect to a point target somewhere within the cone
which represents the field of view of the seeker.

The
technique
is
based on the idea of mechanically chopping the light
flux which impinges on a detector, in such a fashion that the
characteristics of the chopped light pulses vary with the position of
the light source in the field of view. Because the detector produces an
electrical signal directly proportional to the impinging light flux,
electronic hardware can be built to extract a positional error signal
in
x/y coordinates, suitable for driving a missile autopilot (or other
tracking device).

The
simplest
strategy
for designing a rotating reticle seeker is the
Amplitude Modulation technique, such seekers being commonly referred to
as AM seekers. In an AM seeker of conventional design, the light
collected by a mirror system is focussed to a spot on the detector. In
between the detector and optics lies a whirling disc of optically
suitable (transparent) material, which has translucent and opaque
patterns etched on its surface, to interrupt the flux of infrared light.

In
an
AM
seeker, one half of the disc is translucent, and the other
half covered by a spoke pattern, radiating from the centre of the disc.
The result of this is pattern is an optical/electrical signal which is
a
series of pulses, repeating with every revolution of the reticle. The
timing of these pulses with respect to the rotation of the reticle
produces a phase signal which is proportional to the position in one
axis, while the amplitude (size or strength) of the pulses provides an
error signal proportional to the position in the other axis.

The
limitation of the AM seeker lies in the performance of the AM detection
(here x-axis) circuits, as the average signal from the detector becomes
quite weak in one direction thus producing poor tracking performance in
this axis. A scheme to resolve this is what is termed frequency
modulation (FM), whereby the number of spokes varies with the radial
distance from the centre of the reticle. In this fashion a target
closer
to the centre of the reticle produces a smaller number of pulses per
revolution than a target closer to the outer edge of the reticle. As a
result the error signal in the radial axis of the reticle can be
resolved by a frequency discrimination circuit which is locked to a
reference frequency signal produced by the reticle motor.

Practical
seekers
use
a range of variations on these two themes, with
various schemes using fixed cassegrainian mirrors and moving reticles,
or rotating secondary mirrors and fixed reticles, the latter
arrangement
used in the Sidewinder family.

Other
design
issues
in reticle seekers revolve about the detector
element, its supporting optics and cooling system employed. The
detector
is a small piece of semiconductor material with suitable photo-electric
properties, ie it changes its electrical resistance or produces an
electrical current or voltage when illuminated. The key design
parameters in choosing a detector are sensitivity, a measure of how
faint a light signal will generate a useful electrical response, and
colour sensitivity, a measure of which visible or infrared wavelengths
will or will not produce a response. Most semiconductors used for the
purpose have some characteristic longest wavelength to which they
respond, while producing output for all shorter (hotter) wavelengths.
Therefore some detector materials can see only hot objects like
tailpipes, whereas others can see the whole aircraft.

Because
all
hot
objects, such as the sun or flares, emit infrared
blackbody radiation, a missile seeker must have means of reducing or
removing such sources of infrared light to prevent seeker seduction.
Therefore optical filters are used. These filters are typically made of
a rare earth doped glass, with a multiple layer interference filter
deposited on the surface. Such filters are essentially transparent over
a narrow range of colours and opaque to all others, therefore passing
only the desired infrared colour through to the detector.

Cooling
the
detector
is a means of improving its sensitivity. Even the
meagre amount of heat in a detector at room temperature will produce a
response in a good material, resulting in thermal noise which would
mask
the target, therefore the detector must be cooled to prevent this. Two
strategies are typically used for this purpose, thermoelectric cooling
with a Peltier device or gas cooling. A Peltier is a thermocouple which
acts as a heat pump, albeit very inefficient, when electrical current
is
passed through it. Gas cooling relies on the expansion of compressed
gas, and while lighter than Peltier schemes, usually imposes a limit on
total seeker cooling time when the gas bottle is exhausted.

The
evolution
of
heatseeking missiles over the last four decades has
seen almost every one of these schemes, or combinations thereof
employed. The Sidewinder is a good instance.